The present invention relates to a circuit arrangement for generating a direct control voltage which is dependent on an alternating voltage. More particularly, the present invention relates to such a circuit arrangement, particularly for dynamic compression or expansion, including a first charging capacitor which is charged through a first charging circuit and is discharged through a first discharging circuit, and a second charging capacitor which is charged through a second charging circuit and a third charging circuit connected in parallel therewith and is discharged through a second discharging circuit, with the third charging circuit including a controllable current path which is actuated when the voltage of the first charging capacitor exceeds a given threshold value.
German published patent application DE-AS 2,830,784, corresponding to U.S. Pat. No. 4,321,482 issued Mar. 22, 1982 to Ernst Schroder, discloses the use of such a circuit arrangement in compander systems.
For good operation of a compander system it is necessary to have a short decay time so that, after an abrupt end of the useful signal, there will be no noise tails. On the other hand, too short a delay time with low-frequency useful signals leads to high fluctuations in the control voltage and thus to signal distortion through regulation. To avoid such signal distortion, a long decay time is required. These contradictory requirements are met by time constant switching. Such switching is effected at the compander end by the alternating voltage output signal and at the expander end by the alternating voltage input signal.
Such charging time constant switching is realized, for example, by a retriggerable monoflop, as disclosed in "Wissenschaftliche Berichte AEG-TELEFUNKEN" [Scientific Reports from AEG-TELEFUNKEN], volume 52 (1979), page 103.
In this circuit, a capacitor is charged via a current source. One input of a differential amplifier is connected with the capacitor, the other input with a reference voltage. The charging of the capacitor first produces a time delay. If the capacitor voltage finally reaches the level of the reference voltage, the differential amplifier is switched very quickly. This is done in a range of about 50 mVolt. The switching of a regulating time constant effected by this circuit therefore occurs very quickly.
This sudden switching of the time constant becomes audible if, due to tolerances of the time determining capacitor and/or its charging current, the switching moments of the time constants in the compressor and the expander no longer coincide.